We have grown GaAs and AlGaAs on (100) oriented Si substrates by molecular beam epitaxy. The epitaxial growth was studied in situ by reflection high-energy electron diffraction. Low-temperature photoluminescence, Raman scattering, and scanning electron microscopy were used to characterize the epitaxial layers. It is shown for the first time that antiphase disorder could be suppressed. The doped AlGaAs grown directly on Si substrates exhibited PL efficiency similar to that of AlGaAs grown on GaAs substrates.